The present invention relates generally to welding an end plug on a nuclear reactor fuel rod cladding tube, and more particularly, to an apparatus for drawing a vacuum on the interior of a nuclear fuel rod cladding tube when the tube is in position for such welding and also to a system for such welding which incorporates the vacuum apparatus.
The core of a nuclear reactor is composed of a multiplicity of fuel assemblies with each fuel assembly comprised of a plurality of fuel rods. The steps included in the conventional manufacture of a typical nuclear reactor fuel rod started with the loading of nuclear fuel pellets in a cladding tube. Then a bottom end plug was inserted in the bottom of the cladding tube, and a top end plug was inserted in the top of the cladding tube. The top end plug contained an axial bore. The top of the cladding tube with the inserted top end plug was placed in a welding chamber. The axial bore abutted a hole in the rotating stop of the welding chamber. To prevent the atmosphere within the interior of the fuel rod from contaminating the environment of the welding chamber during welding, a vacuum was drawn through the hole in the rotating stop of the welding chamber. This was done by having one end of a vacuum hose attached to a vacuum pump and the other end attached to the hole in the rotating stop. The cladding tube was rotated as the girth weld of the top end plug was made. Then the cladding tube was moved to a separate station for girth welding the bottom end plug. This was done without applying a vacuum to the interior of the cladding tube because the bottom end plug had no axial bore to transmit the atmosphere within the cladding tube to the welding chamber. Then the cladding tube was transported to another station for pressurization of the interior of the fuel rod with helium through the axial bore of the top end plug, and for the closing of the axial bore with a seal weld. The above steps included the use of machines to help perform the various operations.
A common defect that occurred during girth welding was undercutting of the inside diameter of the cladding tube. It was noted that a side benefit of drawing the vacuum on the inside of the fuel rod during girth welding of the top end plug was the minimization of that defect. Therefore, what is needed is a device which will automatically apply a vacuum to the cladding tube during girth welding of the bottom end plug to minimize undercutting of the inside diameter of the cladding tube for this end plug also.